1. Field of the Invention
The present invention relates to an audio signal processing circuit.
2. Description of the Related Art
With the recent progress of miniaturization and thinning of various audio equipment such as thinning of a TV set and miniaturization of a sound reproducing device, speakers for outputting a sound have been also miniaturized.
Accordingly, in order to compensate for insufficient reproduction capability of a low-pitched sound of such a small-sized speaker, a technique has been developed of extracting, from the original audio signal, an audio signal in a range lower than the lowest reproducible frequency of a speaker, generating a harmonic from this audio signal in the low range, and adding this harmonic to the original audio signal to output the result from a speaker (See Japanese Laid-Open Patent Application Publication No. 2005-278158, for example).
When a sound is reproduced by using such a technique, a sound in a low range, which is not actually outputted from the speaker, is heard by a human being as if it were outputted therefrom, thereby being able to improve audibility.
When an audio signal in a low range is extracted from the original audio signal, a low-pass filter is used, but the audio signal in the low range having passed through the low-pass filter has a phase delay according to a frequency.
When a harmonic is generated from this audio signal in the low range that has different phase delays generated according to the frequencies, even if a phase change does not occur in generating a harmonic, the generated harmonic has a phase different according to a frequency similarly to the audio signal before generating the harmonic.
Thus, since this harmonic and the original audio signal are different in phase according to the frequency, a waveform of an audio signal generated by adding these signals is distorted, resulting in a factor of deterioration in sound quality of a sound outputted from the speaker.
That is, the harmonic generated from the audio signal in a range lower than the lowest reproducible frequency of the speaker is added to the original audio signal and the result is outputted, thereby being able to reproduce a sound with good audibility with a low-pitched sound being emphasized, however, deterioration in the sound quality is caused by distortion of the waveform of the audio signal.